This invention relates to thermally activated electron-emissive cathodes for electron guns used in television picture tubes, and in particular to an improved process for coating such cathodes with electron-emissive material in manufacture.
Desired characteristics in cathodes include maximum emission with maximum life, resistance to "poisoning" from contaminants, and simplicity of manufacture, with a high yield.
To provide maximum emission and life, a cathode should be microporous; that is, have a relatively smooth surface and be uniformly dense throughout. A uniform density is particularly important with regard to long life; if the cathode coating is microporous, hot spots may develop during cathode emission, and stresses may be induced within the coating which may result in the eventual flaking off of significant portions of the coating, with a consequent reduction in both emission and life of the cathode.
Uniform density and microporosity are also factors in the resistance of the cathode to poisoning by contaminants. Cathode coatings having a rough surface and large expose much larger areas of the cathode coating to the effect of contaminants. The less surface area that is exposed to contaminants, the more resistant the cathodes will be to poisoning. The cathode that is poisoned will have a reduced emission level, or emit for only a short time. Because the cathode often will not fail until after the cathode ray tube has been sealed, or in cases of short life, after it is installed in the product, the effect of a relatively early failure can be troublesome and costly.
Another desired characteristic is that the cathode coating must be easy to apply during manufacture, and the process must provide a high yield. Manufacturing costs can be greatly reduced if the process steps are few, if standard equipment including automated equipment easily adapted to production can be used, and if relatively unskilled personnel can be assigned to the process.
Resistance to poisoning from contaminants is also a factor in reducing costs of manufacture. The manufacturing environment may be high in level of contaminants, particularly with regard to the halogens and the byproducts of such process steps as the cleaning and degreasing of parts. If the cathode coating is resistant to poisoning from such contaminants, precautions required to protect the cathodes from contamination can be made relatively simple, without the need for costly enclosures and elaborately controlled environments.
Prior art methods for coating cathodes comprise, in general, the steps of inserting the cathodes into a suitable holding fixture and causing the cathodes to repetitively pass a spray gun spraying a suspension of electron-emissive compound. More than one such coat may be applied. Such methods, while satisfactory to a certain extent, have proved to be less than adequate in meeting the objectives of uniform density, resistance to contaminants and high yield achieved by the present invention.